Manual Steering Vs. Power Steeringby Richard Rowe
Although manual steering systems are simple, reliable and beloved by enthusiasts, power steering systems are equally prized for their ease of use and versatility. Although the wallowing slush boxes of old are still used in many economy cars, modern technology has enhanced those hydraulic-assisted racks so that some offer almost the same response and performance of a manual rack with none of the hassles.
A manual steering rack uses a rack and pinion to turn the rotational movement of the steering wheel into the back-and-forth movement required to turn the wheels. The pinion is a round gear connected to the steering column; the pinion engages the rack, which is a flat bar with gear teeth cut into the top. A hydraulic power steering system uses an engine-mounted pump to pressurize a two-way ram, which helps to push or pull the rack in one direction or the other.
There are two basic kinds of rack: linear and variable rate. A linear rack has the same number of teeth all the way through, so the wheels respond the same to steering input regardless of angle. A variable-rate rack uses very closely spaced teeth in the center for fine adjustments while the wheel is pointed relatively straight. The widely spaced teeth on either end quicken the ratio to ease parking. Variable-rate racks are better suited to power-assisted steering, because the quicker ratio makes the wheel harder to turn.
Speed and Precision
Generally speaking, power steering systems respond quicker to steering input than manual boxes. The manual box's lack of assist means that the gear ratio must be lower to allow the driver to turn the wheel. This lower ratio equates to more turns lock-to-lock, so steering response from a manual box is generally much lower. However, manual steering is inherently more precise because it requires more steering input to move the wheels.
Feedback is an often overlooked but crucial facet of steering performance. Feedback is the resistance to wheel movement, which is dictated by front wheel traction. For example, the steering wheel turns much more easily on ice than on dry pavement. Feedback in the form of resistance to movement sends an almost subconscious signal to your brain, giving it vital information as to how well the front wheels are gripping the pavement. This is especially crucial in front-wheel drive cars, which rely on but two tires to do all of the steering and acceleration, and most of the braking. By design, power steering systems reduce steering resistance, and thus feedback. For this reason, most race cars run a manual steering rack.
Manual steering racks have changed very little over the years, but power steering has had a host of improvements. Most of these have been geared toward giving the power rack all of the precision and feedback of a manual rack while maintaining the power system's ease of use. These enhancements include speed-variable power assist (which offers more assistance at low speed) like that found in the Honda S2000, electrically assisted power steering (which varies power according to need and steering angle), and hybrid electric/hydraulic systems that use a computer-controlled electric motor to power the pump.